1. Field of the Invention
The present invention relates to a transconductor, and more particularly, to a method of acquiring a low distortion and high linear characteristic of a triode-typed transconductor and the triode-typed transconductor having low distortion and high linear characteristic.
2. Description of the Related Art
A transconductor (operational transconductance amplifier; OTA) is a circuit that converts an input voltage into a proportional output current, and is used variously in a lot of electric systems such as an active filter, an analog/digital converter, a delta-sigma modulator, an integrator, and/or a gyrator. Generally, the transconductor should have following characteristics. First, the transconductor should have a very large input/output impedance in order to reduce a load effect. Second, the transconductor should have indefinite operational frequency band in order to maintain amplitude and phase of the output current regardless of the input frequency. Third, the voltage-current conversion should be tuned easily in a wide range. Fourth, a common mode rejection ration (CMRR) should be high so that the output DC voltage is adjacent to the input DC voltage. Fifth, the output current should be output in a predetermined proportion to the input voltage within a linear input voltage range. The wide output current swing range in proportion to the input voltage is an important parameter for determining the characteristics of the transconductor. Especially, in order to satisfy desired a signal to noise (S/N) ratio and a distortion ratio in an electric system, the linear input/output ranges should be wide and should be maintained constantly within an error range in the linear range.
There are a lot of linearizing methods of the transconductor, and the transconductor including a complementary metal-oxide semiconductor (CMOS) transistor can be classified into three kinds. That is, a transistor operating in a saturation region, a transistor operation in a triode region, or a transistor combining above two can be used.
FIGS. 1 through 3 are circuit diagrams illustrating various structures of the conventional operational transconductance amplifier (OTA) realized using metal-oxide semiconductor (MOS) transistors. Reference numerals M1, M2, M3, and M4 denote MOS transistors.
The transconductor shown in FIG. 1 has a degenerated differential-pairs structure, in which resistance components (R) that can be tuned are connected to saturation region input transistors M1 and M2 in order to maintain the linearity. However, if the tunable resistance components R are smaller than the impedance 1/Gm1,2 of sources of input transistors M1 and M2, the linearity can be degraded.
The transconductor shown in FIG. 2 is formed in a triode-typed structure, and uses the MOS transistor operating in the triode region, which shows linear current characteristic with respect to the input voltage, as the input transistors M1 and M2. Drain-source voltages of the input transistors M1 and M2 are fixed as small values so that the transistor can be in the triode region, and the transconductance value and tuning region are decided by the value. However, the linearity can be affected by change in the drain-source voltage and mobility (μ).
The transconductor shown in FIG. 3 has a square-law structure, and further includes additional MOS transistors M3 and M4 instead of the resistance components R in the structure of the transconductor in FIG. 1. All of the transistors M1, M2, M3, and M4 operate in the saturated states, the distortion can be reduced and the linearity can be maintained by feedback of harmonic component. There are many other transconductors besides above three structures, however, these are modifications of the above structures.
The transconductor of the degenerated differential-pairs structure in FIG. 1 and the transconductor of square-low structure in FIG. 3 are favorable to obtain high operational speed and high gain, however, few input gates that are connected to each other in parallel should be combined in order to expand the linear input range and reduce the distortion components. The above structure causes complexity in the circuit, consumption of large area, and power consumption, and is not suitable for low voltage circuit since transconductor operates in the saturation region. Moreover, the linear region is narrower than that of transistor operating in the triode region, and the error range of the transconductance is large even in the linear range.
FIG. 4 is a circuit diagram illustrating a part of the triode-typed transconductor that is generally used as the linear transconductor and uses a gain-boosting amplifier feedback.
As described above, the transconductor structure shown in FIG. 2 has higher linear characteristic than those of the transconductor structures shown in FIGS. 1 and 3. As shown in FIG. 4, a node voltage at a drain node (a) in the first MOS transistor M1, that is, the drain-source voltage Vds is maintained constantly by an amplifier 402. Ideally, an output current Io that is in proportion to the input voltage Vin is generated in the triode region T where the input voltage Vin is larger than a sum of a threshold voltage Vth of the first MOS transistor M1 and the node voltage Vds (Vth+Vds). Generally, the transconductance Gm can be represented as following Equation 1.Gm=μ·Cox·(W/L)M1·Vds  (1),where μ denotes the mobility, Cox denotes a gate metal-oxide capacitance in the first MOS transistor M1, and W/L denotes a ratio between width and length of the first MOS transistor M1.
According to Equation 1, the transconductor having very large linear input/output characteristics can be designed when the node voltage Vds is maintained as a constant in a state of triode region operating mode.
In the triode-typed transconductor structure, the input DC voltage Vin that is input into a gate terminal of the first MOS transistor M1 should increase in order to obtain a large linearity range. However, when the gate input DC voltage Vin is actually increased, the constant transconductance cannot be maintained any more. Because, when the gate voltage increases, the intensity of electric field in a vertical direction toward the channels under the gate insulating layer is strengthened, and accordingly, inversion carriers that are pulled toward the gate insulating layer are bounded toward the channels by an energy wall of the gate insulating layer, that is, scattering between the inversion carriers and the gate insulating layer is generated and drift velocity of the carriers is reduced. Therefore, although the mobility μ in Equation 1 is the constant in an ideal case, the mobility μ is not the constant in actual process. The actual mobility μ can be represented as following Equation 2.
                    μ        =                              μ            0                                                                      ⁢                          1              +                                                (                                                            U                      a                                        +                                                                  U                        c                                            ·                                              V                                                  bs                          ,                          eff                                                                                                      )                                ·                                  (                                                                                    V                        gs                                            +                                              V                        th                                                                                    T                      ox                                                        )                                            +                                                U                  b                                ·                                                      (                                                                                            V                          gs                                                +                                                  V                          th                                                                                            T                        ox                                                              )                                    2                                                      ⁢                                                                                    (        2        )            
As shown in Equation 2, the mobility μ is a function of the gate-source voltage Vgs, and especially, is a variable that is reduced as the gate-source voltage Vgs increases.
Thus, in the actual process, the output current lo does not increase linearly according to the increase of the input voltage Vin, but reduces, as shown in FIG. 6. Like, the transconductance Gm is not maintained constantly, but is reduced. In a case where the input voltage is input in the differential-pairs input method, the linearity may be improved higher than that of single input method. However, the perfect linearity cannot be obtained in the differential-pairs input method, since the non-linearity characteristic is still caused by the variation of the mobility μ due to the gate input voltage Vin.